The present invention relates to an apparatus and method for image exposure used in a CTP (Computer to Plate) system, a film setter subjecting a lith film to image exposure, or an apparatus for manufacturing a printed circuit board (hereafter referred to as “PCB”), in which a wavelength conversion ultraviolet laser (hereafter referred to as “wavelength conversion UV laser”) of a pulse-driven type is employed.
Currently, regarding a wavelength conversion UV laser, it is generally considered that, in order to subject an infrared (hereinafter referred to as “IR”) laser beam to nonlinear wavelength conversion at high efficiency, the IR laser is desirably pulse-driven to increase the peak power. Under the present circumstances, an IR laser which has a pulse repetition frequency of 80 MHz (a pulse repetition period of 12.5 nanoseconds (ns)) and has a pulse width of 12 picoseconds (ps) is used for image exposure of a presensitized plate (PS plate) or the like.
When the above-mentioned IR laser is used for the image exposure of a PS plate or the like, if the pulse repetition period (pulse repetition frequency) interferes with the image exposure period (data frequency) of the scanning exposure, it becomes impossible to carry out a proper image exposure. Then, the image exposure period and the pulse repetition period need to be synchronized.
However, the IR laser which is currently used for the image exposure, as described above, has a high pulse repetition frequency of 80 MHz, and has a very narrow pulse width of 12 picoseconds.
For this reason, such a high performance as a sampling frequency of about 100 GHz is required for a photoelectric conversion device used for synchronization with the pulse repetition period of the IR laser, and an electronic circuit for generating a synchronous signal based on the output from the photoelectric conversion device. As a result, there is a problem in that the photoelectric conversion device and the electronic circuit become expensive to increase the total cost.
Then, there is proposed a technique for preventing a loss of synchronism between the image exposure period and the pulse repetition period of a pulsed laser by increasing the pulse repetition period (refer to JP 2002-523905 A for example).
The laser repetition rate multiplier (hereinafter referred to as “repetition multiplier”) disclosed in JP 2002-523905 A is a device in which, after one beam emitted from a pulsed infrared laser is split into a plurality of split beams to which are given different delay times, respectively, the split beams are combined into a single beam, thereby reducing the peak power of the resultant beam to increase the pulse repetition period. In the repetition multiplier disclosed in JP 2002-523905 A, a pulse repetition frequency of 80 MHz is, for example, quadrupled to 320 MHz, thereby suppressing the interference between the pulse repetition period and the image exposure period of the scanning exposure.
However, in the case of the repetition multiplier disclosed in JP 2002-523905 A, optical path differences are provided using a plurality of mirrors to give the different delay times to the split beams, respectively, thereby delaying the split beams. For this reason, there is a problem in that the device construction becomes complicated.
In addition, in order to combine the split beams, high assembly precision is required for the constituent elements such as a lens and a mirror. As described above, the repetition multiplier disclosed in JP 2002-523905 A involves a problem in that since the device construction becomes complicated and the high assembly precision is also required, the cost is increased.
Moreover, for equalization of the peak output of all the split beams, the reflectivity and the transmittance of the mirrors need to be set to their respective, predetermined values. Thus, a high quality is required also for the elements constituting the device.